Seagrass Roots Research Articles

Eutrophication indirectly reduced carbon sequestration in a tropical seagrass bed

Seagrass beds occur globally and are substantial blue carbon sinks, yet factors that affect sediment organic carbon (SOC) sequestration are poorly characterized. We investigated the influences of eutrophication on SOC sequestration capacity in a mixed seagrass bed dominated by Thalassia hemprichii and Enhalus acoroides. We surveyed a tropical seagrass bed in the South China Sea along fish farming-induced gradients of nutrients to assess the variability of SOC content and composition in 30-cm-long sediment cores. No significant difference was observed for the SOC content and stock between the two species. Significantly higher microbial biomass carbon and SOC density were observed at the surface 3 cm layer under E. acoroides, and the longer roots of E. acoroides enhanced the seagrass contribution to the SOC near the root tip. SOC had a statistically lower concentration in the sediment layers where the seagrass roots primarily thrive close to fish farming, but greater exchangeable organic carbon was found in all the sediment layers as they approached fish farming. Nutrient enrichment markedly induced a larger ratio of labile C pools in almost all sediment layers. The mean SOC stock in the seagrass bed in Xincun Bay was 6.80 ± 1.03 Mg C/ha, and it showed insignificant variation along the nutrient gradient. Eutrophication may indirectly reduce the SOC sequestration capacity in the seagrass bed by enhancing labile organic carbon with a shorter resident time, and measures should be taken to reduce nutrient input into the seagrass bed to enhance its carbon sink.

Accounting for black carbon lowers estimates of blue carbon storage services

The canopies and roots of seagrass, mangrove, and saltmarsh protect a legacy of buried sedimentary organic carbon from resuspension and remineralisation. This legacy’s value, in terms of mitigating anthropogenic emissions of CO2, is based on total organic carbon (TOC) inventories to a depth likely to be disturbed. However, failure to subtract allochthonous recalcitrant carbon overvalues the storage service. Simply put, burial of oxidation-resistant organics formed outside of the ecosystem provides no additional protection from remineralisation. Here, we assess whether black carbon (BC), an allochthonous and recalcitrant form of organic carbon, is contributing to a significant overestimation of blue carbon stocks. To test this supposition, BC and TOC contents were measured in different types of seagrass and mangrove sediment cores across tropical and temperate regimes, with different histories of air pollution and fire together with a reanalysis of published data from a subtropical system. The results suggest current carbon stock estimates are positively biased, particularly for low-organic-content sandy seagrass environs, by 18 ± 3% (±95% confidence interval) and 43 ± 21% (±95% CI) for the temperate and tropical regions respectively. The higher BC fractions appear to originate from atmospheric deposition and substantially enrich the relatively low TOC fraction within these environs.

Low Light Availability Alters Root Exudation and Reduces Putative Beneficial Microorganisms in Seagrass Roots.

Seagrass roots host a diverse microbiome that is critical for plant growth and health. Composition of microbial communities can be regulated in part by root exudates, but the specifics of these interactions in seagrass rhizospheres are still largely unknown. As light availability controls primary productivity, reduced light may impact root exudation and consequently the composition of the root microbiome. Hence, we analyzed the influence of light availability on root exudation and community structure of the root microbiome of three co-occurring seagrass species, Halophila ovalis, Halodule uninervis and Cymodocea serrulata. Plants were grown under four light treatments in mesocosms for 2 weeks; control (100% surface irradiance (SI), medium (40% SI), low (20% SI) and fluctuating light (10 days 20% and 4 days 100%). 16S rDNA amplicon sequencing revealed that microbial diversity, composition and predicted function were strongly influenced by the presence of seagrass roots, such that root microbiomes were unique to each seagrass species. Reduced light availability altered seagrass root exudation, as characterized using fluorescence spectroscopy, and altered the composition of seagrass root microbiomes with a reduction in abundance of potentially beneficial microorganisms. Overall, this study highlights the potential for above-ground light reduction to invoke a cascade of changes from alterations in root exudation to a reduction in putative beneficial microorganisms and, ultimately, confirms the importance of the seagrass root environment – a critical, but often overlooked space.

Identifikasi Kandungan Senyawa Fitokimia Pada Daun dan Akar Lamun di Pantai Samuh Bali

Seagrass is the only flowering plant which has a true roots, leaves, and stems, an angiosperm plant that adapted to live entirely in the ocean. Seagrass produces several compounds which produced through secondary metabolism, one of the secondary metabolism is the phytochemicals. Seagrass ecosystem in Tanjung Benoa was discovered along Samuh Beach, where 9 kinds of seagrass species were found around the seagrass ecosystem in Tanjung Benoa. Samuh Beach has a tranquil water conditions and sandy substrate. The activities of marine tourism and hotel waste disposal in Tanjung Benoa waters cause major ecological pressure and physical pressure for the seagrass ecosystem. This study aims to determine the content of phytochemicals in the leaves and roots of seagrass. Screening method was used to determine the content of bioactive alkaloids, flavonoids, saponins, steroids, and tannin compounds. Each of these compounds has an important role in the seagrass. The existence of chemical compounds of the flavonoid, alkaloid and steroid groups in the roots and leaves of Cymodocea rotundata seagrass, Enhalus acoroides, Thalassia hemprichi, Halophila ovalis, Halophila minor, Halodule uninervis and Sryngodium isoetifolium in this study indicate that the seven species of seagrass has potential as a natural chemical antifouling, antifungal, and antibacterial.Then the seagrass will be protected from the danger of predators or epiphanies that interfere with the growth of seagrass and the ecosystems on Samuh Beach will remain intact.

Push‐pull incubation method reveals the importance of denitrification and dissimilatory nitrate reduction to ammonium in seagrass root zone

In this study, we developed a push-pull incubation method to measure denitrification and dissimilatory nitrate reduction to ammonium (DNRA) in subsurface sediments of subtidal seagrass meadows. This subtidal push-pull technique, adapted from a push-pull method developed for intertidal salt marshes, used mini-piezometers to directly sample pore water in the root zone of the seagrass during an in situ incubation. The pore water was amended with 15 NO3− and with Ar(g) as a tracer gas, and the denitrification products (28N2(g), 29N2(g), and 30N2(g)) were measured with membrane inlet mass spectrometry (MIMS), using the Ar tracer to correct for dilution and gas loss. Production of 15 NH4+ was also measured using hypobromite oxidation and MIMS to determine rates of DNRA. Using this new technique, subsurface rates of denitrification and DNRA were determined to be 17.5 μmol N m−2 h−1 and 14.7 μmol N m−2 h−1, respectively, for a restored Zostera meadow. Rates showed substantial spatial variability, likely due to both heterogeneous conditions in the root zone sediment and variable in situ conditions. When compared to traditional core and slurry incubations, push-pull rates were greater and more variable, suggesting that the push-pull technique more accurately captures heterogeneity and the natural range of denitrification and DNRA in subsurface sediments under field conditions. In vegetated systems with low water column nitrate concentrations, the majority of denitrification and DNRA occurs below the sediment surface, and the subtidal push-pull technique provides an effective method to assess these subsurface rates.

Caribbean Spiny Lobster Fishery Is Underpinned by Trophic Subsidies from Chemosynthetic Primary Production

The Caribbean spiny lobster, Panulirus argus, is one of the most valuable fisheries commodities in the Central American region, directly employing 50,000 people and generating >US$450 million per year [1]. This industry is particularly important to small island states such as The Bahamas, which exports more lobster than any other country in the region [1]. Several factors contribute to this disproportionally high productivity, principally the extensive shallow-water banks covered in seagrass meadows [2], where fishermen deploy artificial shelters for the lobsters tosupplement scarce reef habitat [3]. The surrounding seabed communities are dominated by lucinid bivalve mollusks that live among the seagrass root system [4, 5]. These clams host chemoautotrophic bacterial symbionts in their gills that synthesize organic matter using reduced sulfur compounds, providing nutrition to their hosts [6]. Recent studies have highlighted the important role of the lucinid clam symbiosis in maintaining the health and productivity of seagrass ecosystems [7, 8], but their biomass also represents a potentially abundant, but as yet unquantified, food source to benthic predators [9]. Here we undertake the first analysis of Caribbean spiny lobster diet using a stable isotope approach (carbon, nitrogen, and sulfur) and show that a significant portion of their food (∼20% on average) is obtained from chemosynthetic primary production in the form of lucinid clams. This nutritional pathway was previously unrecognized in the spiny lobster's diet, and these results are the first empirical evidence that chemosynthetic primary production contributes to the productivity of commercial fisheries stocks.

The effect of increasing seagrass root complexity and redox potential on the population of Pillucina vietnamica (Bivalvia: Lucinidae) in southwestern Thailand

ABSTRACTIt is well known that seagrasses provide habitat to many bivalves. Pillucina vietnamica is the dominant bivalve (> 4000 inds m−2) in seagrass beds at the Haad Chao Mai National Park, Thailand, where seagrasses are abundant and diverse. This study aimed to determine the relationship of root, rhizome and redox potential to the P. vietnamica population. The study was performed using patches of three seagrass species (Halophila ovalis, Thalassia hemprichii and Cymodocea rotundata), which have different root morphologies and occur in both mono- and multi-specific stands as well as adjacent to areas of bare sand. The results showed that, within the vegetated areas, the abundance of P. vietnamica increased with increased root complexity and redox potential. A high abundance of P. vietnamica was found in patches of the branched root species, C. rotundata, suggesting that the root mats of this species was a preferred habitat for bivalves by providing a complex habitat and sufficient oxygen levels. The size class frequency of the bivalves differed between seasons. They were smaller and distributed bimodally in the summer but were larger and distributed unimodally in the monsoon month, suggesting a shift from the juvenile stage to the mature stage.

In vitro and in vivo efficacy of partially purified herbal extracts against bacterial fish pathogens

The effect of partially purified extracts of sea grass (Cymodocea serrulata), turmeric (Curcuma longa), Spirulina (Spirulina platensis), and beet root (Beta vulgaris) on the antimicrobial, antioxidant, and hemato-biochemical changes in ornamental dotty back, Pseudochromis dilectus against isolated fish bacterial pathogens (Proteus mirabilis, Bacillus pumilus, and Staphylococcus sciuri) under in vitro and in vivo conditions is reported. Among the extracts turmeric extract exhibited significant antibacterial activity. The extract possesses the highest antioxidant property and phenol content; the GC–MS analysis revealed the highest presence of Ar-turmerone (42.85%) and Curlone (18.80%). The potential fractions possess reducing power and free radical scavenging activity. The mortality against P. mirabilis at doses of 2.5×108 and 2.5×108CFU was 100% whereas against S. sciuri (2.5×107 and 2.5×108) it was 70 and 80%. The highest bacterial load was observed in kidney (3.1×107CFU/g) and liver (2.6×108CFU/g) against P. mirabilis. The LD50 study suggested that all the fish die at doses of 2.5×107 (CFU/fish) and 2.5×108 (CFU/fish) after 15days of challenge with P. mirabilis. Among the purified sub fractions, the sub-fraction II (F-II) at 5 and 10mg/ml resulted in elevated levels of WBC and RBC, lymphocytes, and neutrophils; further the percentage of monocytes and granulocytes, total protein, total lipid, and glucose also increased significantly. At 1mg/ml the relative percent survival (RPS) was 100% in sub-fraction II (F-I) group as in control. With 5mg/ml in F-I and 1mg/ml in F-II group the RPS was 94%; however, in 10mg/ml of F-I and 5mg/ml of F-II groups the RPS declined to 89%. The lowest mortality of RPS (79%) was observed in 10mg/ml in F-II group. The present results suggest that the partially purified turmeric extract elevated antimicrobial and antioxidant activities by improving the hemato-biochemical profiles and survival rate in P. dilectus against P. mirabilis. Statement of relevanceThe success of any fish culture depends on the health status of cultivated species. Fish diseases are one of major threats in aquaculture industry inflicting heavy mortality in the world. To prevent and control of the diseases traditionally was used several antibiotics or chemicals during the last few decades. However, these methods have been criticized for their negative impacts such as accumulation of drugs in tissues, development of drug resistance, and immunosuppression. Several herbal and their active principles possess better antioxidant activity, phenols content, reducing power, and free radical scavenging activity. Therefore in the present study was isolated new fish pathogen from the infected fish and to investigated antioxidant activity and their pathogenicity and hematological changes in dotty back, P. dilectus against P. mirabilis. This work is novel in aquaculture to design or prepare cost effective drug or feed for fishes.

Vertical distribution and respiration rates of benthic foraminifera: Contribution to aerobic remineralization in intertidal mudflats covered by Zostera noltei meadows

The present study investigates the influence of seagrass root systems on benthic hard-shelled meiofauna (foraminifera). In February and July 2011, sediment cores were collected at low tide at two sites in Arcachon lagoon, a vegetated site with Zostera noltei and a second site with bare sediments. We used the highly discriminative CellTracker™ Green fluorogenic probe technique to recognize living foraminifera and to describe foraminiferal density and diversity. Three dominant species of foraminifera were observed: Ammonia tepida, Haynesina germanica and Eggerella scabra. The two calcareous species, A. tepida and H. germanica, were preferentially found in the upper half to 1 cm of the sediment. At the vegetated site, these two species had a slightly deeper microhabitat. In the literature, both species have been described alive in much deeper sediment layers, possibly due to false positives from the Rose Bengal staining method. These two species also showed 1) higher densities at the site with Z. noltei, 2) a higher density in February when conditions were supposed optimal due to a microphytobenthos bloom, and 3) dissolved calcitic shells in July, probably resulting from a lower pH. The agglutinated species E. scabra was present alive down to at least 7 cm depth. E. scabra showed high densities in the anoxic part of the sediment at both the vegetated and bare sites, with a substantially higher density in summer at the site with bare sediments. Its presence at depth may be related to its trophic requirements; this species could be less dependent on labile organic matter than A. tepida and H. germanica. On this intertidal mudflat, the foraminiferal contribution to aerobic carbon remineralization, based on respiration rate measurements, can account for up to 7% of the diffusive oxygen uptake, almost five times more than the maximum contribution recorded in open marine environments (300 m depth) in the Bay of Biscay.

N-Alkanes and stable C, N isotopic compositions as identifiers of organic matter sources in Posidonia oceanica meadows of Alexandroupolis Gulf, NE Greece

We analyzed n-alkane contents and their stable carbon isotope composition, as well as the carbon and nitrogen isotope composition (δ13C, δ15N) of sediment organic matter and different tissues of Posidonia oceanica seagrass sampled in Alexandroupolis Gulf (A.G.), north-eastern Greece, during 2007–2011. n-Alkane contents in P. oceanica and in sediments showed similar temporal trends, but relative to bulk organic carbon content, n-alkanes were much more enriched in sediments compared to seagrass tissue. Individual n-alkanes in sediments had similar values than seagrass roots and rhizomes and were more depleted in 13C compared to seagrass leaves and sheaths, with δ13C values ranging from −35‰ to −28‰ and from −25‰ to −20‰, respectively. n-Alkane indexes such as the Carbon Preference Index, carbon number maximum, and n-alkane proxy 1 (C23+C25/C23+C25+C29+C31) indicate strong inputs of terrestrial organic matter, while the presence of unresolved complex mixtures suggests potential oil pollution in some sampled areas.

Species-specific effects of seagrass on belowground biomass, redox potential and Pillucina vietnamica (Lucinidae)

The network of roots and rhizomes of seagrasses plays an important role in coastal ecosystems by anchoring the substrate firmly, controlling the biogeochemistry of the sediments and providing habitat for many infaunal organisms. The purpose of this study was to compare five seagrass communities in southern Thailand in terms of species composition, belowground biomass, redox potential and the abundance of the dominant bivalve Pillucina vietnamica. The communities of Halophila ovalis, Cymodocea rotundata, Thalassia hemprichii, H. ovalis with C. rotundata and C. rotundata with T. hemprichii were studied. Belowground biomass, redox potential, bivalve abundance and bivalve size differed among communities and depths (P < 0.05). The redox potential in vegetated areas was lower than in bare sand, showing that seagrass roots trapped more organic matter than did bare sand. The lowest redox potential was in a monospecific patch of the branching root species, C. rotundata. The abundance of P. vietnamica in vegetated areas was lower than in bare sand (P < 0.05). Pillucina vietnamica was relatively large in size in the monospecific stands of C. rotundata and T. hemprichii, where relatively high root biomass and belowground biomass, respectively, were found. Our results suggest that seagrass promotes anoxic conditions belowground by trapping sediment but could obstruct the burrowing activity of lucinids, as dense populations of lucinids occurred mainly in bare sand. However, seagrass also provided shelter for lucinids, as larger bivalves were found in seagrass areas. The interactions among belowground seagrass parts, the biogeochemistry of sediments and the lucinids are highly complex and merit further study.

Communities of Cultivable Root Mycobionts of the Seagrass Posidonia oceanica in the Northwest Mediterranean Sea Are Dominated by a Hitherto Undescribed Pleosporalean Dark Septate Endophyte.

Seagrasses, a small group of submerged marine macrophytes, were reported to lack mycorrhizae, i.e., the root-fungus symbioses most terrestrial plants use for nutrient uptake. On the other hand, several authors detected fungal endophytes in seagrass leaves, shoots, rhizomes, and roots, and an anatomically and morphologically unique dark septate endophytic (DSE) association has been recently described in the roots of the Mediterranean seagrass Posidonia oceanica. Nevertheless, the global diversity of seagrass mycobionts is not well understood, and it remains unclear what fungus forms the DSE association in P. oceanica roots. We isolated and determined P. oceanica root mycobionts from 11 localities in the northwest Mediterranean Sea with documented presence of the DSE association and compared our results with recent literature. The mycobiont communities were low in diversity (only three species), were dominated by a single yet unreported marine fungal species (ca. 90 % of the total 177 isolates), and lacked common terrestrial and freshwater root mycobionts. Our phylogenetic analysis suggests that the dominating species represents a new monotypic lineage within the recently described Aigialaceae family (Pleosporales, Ascomycota), probably representing a new genus. Most of its examined colonies developed from intracellular microsclerotia occupying host hypodermis and resembling microsclerotia of terrestrial DSE fungi. Biological significance of this hitherto overlooked seagrass root mycobiont remains obscure, but its presence across the NW Mediterranean Sea and apparent root intracellular lifestyle indicate an intriguing symbiotic relationship with the dominant Mediterranean seagrass. Our microscopic observations suggest that it may form the DSE association recently described in P. oceanica roots.

Anatomically and morphologically unique dark septate endophytic association in the roots of the Mediterranean endemic seagrass Posidonia oceanica.

Roots of terrestrial plants host a wide spectrum of soil fungi that form various parasitic, neutral and mutualistic associations. A similar trend is evident in freshwater aquatic plants and plants inhabiting salt marshes or mangroves. Marine vascular plants (seagrasses), by contrast, seem to lack specific root-fungus symbioses. We examined roots of two Mediterranean seagrasses, Posidonia oceanica and Cymodocea nodosa, in the northwestern Mediterranean Sea for fungal colonization using light and scanning and transmission electron microscopy. We found that P. oceanica, but not C. nodosa, is regularly associated with melanized septate hyphae in a manner resembling colonization by the ubiquitous dark septate endophytes (DSE) in roots of most terrestrial plants. P. oceanica roots were found to be colonized by sparse dematiaceous running hyphae as well as dense parenchymatous nets/hyphal sheaths on the root surface, intracellular melanized microsclerotia and occasionally also intra- and intercellular hyphae. The colonization was most prominent in the thick-walled hypodermis of the thinnest healthy looking roots, and the mycobiont seemed to colonize both living and dead host cells. Dark septate hyphae infrequently occurred also inside rhizodermal cells, but never colonized vascular tissues. The biological significance of this overlooked marine symbiosis remains unknown, but its morphology, extent, distribution across the NW Mediterranean Sea and absence in C. nodosa indicate an intriguing relationship between the dominant Mediterranean seagrass and its dark septate root mycobionts.

Simultaneous 2D Imaging of Dissolved Iron and Reactive Phosphorus in Sediment Porewaters by Thin-Film and Hyperspectral Methods

This study presents a new approach combining diffusive equilibrium in thin-film (DET) and spectrophotometric methods to determine the spatial variability of dissolved iron and dissolved reactive phosphorus (DRP) with a single gel probe. Its originality is (1) to postpone up to three months the colorimetric reaction of DET by freezing and (2) to measure simultaneously dissolved iron and DRP by hyperspectral imaging at a submillimeter resolution. After a few minutes at room temperature, the thawed gel is sandwiched between two monospecific reagent DET gels, leading to magenta and blue coloration for iron and phosphate, respectively. Spatial distribution of the resulting colors is obtained using a hyperspectral camera. Reflectance spectra analysis enables deconvolution of specific colorations by the unmixing method applied to the logarithmic reflectance, leading to an accurate quantification of iron and DRP. This method was applied in the Arcachon lagoon (France) on muddy sediments colonized by eelgrass (Zostera noltei) meadows. The 2D gel probes highlighted microstructures in the spatial distribution of dissolved iron and phosphorus, which are most likely associated with the occurrence of benthic fauna burrows and seagrass roots.

Effects of tidal inundation on benthic macrofauna associated with the eelgrass Zostera muelleri

Processes, such as sea level rise, that alter tidal inundation regimes have the potential to modify the structure of seagrasses and their dense and diverse faunal communities. This study tested the hypothesis that seagrass-dwelling invertebrate communities would vary across a tidal inundation gradient as a result of direct effects of tidal inundation and indirect effects, arising from changes in seagrass morphology across this gradient. First, we conducted mensurative sampling across tidal inundation gradients to assess how above- and below-ground seagrass biomass, and epi- and infaunal invertebrate communities co-varied with depth. Second, we ran a manipulative field experiment, utilising artificial seagrass rhizomes of varying morphologies, to separate out direct effects of tidal inundation on infaunal communities from indirect effects arising from changes in seagrass root morphology. Mensurative sampling revealed that the abundance and taxon richness of seagrass epi- and infauna, and the above- and below-ground biomass of seagrass each increased with depth across a tidal elevation gradient extending from the high intertidal to the shallow subtidal. The manipulative experiment revealed that the relative importance of direct and indirect effects of tidal inundation in determining the distribution and abundance of infauna were taxon-specific. In general, however, the facilitative effects of rhizome structure were more evident at the intertidal compared to the subtidal elevation. Our results indicate that changes to tidal inundation regime will affect seagrass-dwelling macroinvertebrates through a combination of direct and indirect effects. Therefore, future changes in tidal inundation should be taken into account in developing conservation plans for protecting seagrasses and the biodiversity they sustain.

Diurnal shifts in co-distributions of sulfide and iron(II) and profiles of phosphate and ammonium in the rhizosphere of Zostera capricorni

High resolution, two dimensional distributions of porewater iron(II) and sulfide were measured, using colourimetric DET (diffusive equilibration in a thin film) and DGT (diffusive gradients in a thin film) techniques, respectively, in Zostera capricorni colonised sediments under both light and dark conditions. Low resolution depth profiles of ammonium and phosphate were measured using conventional DET and DGT methods, respectively. Porewater iron(II) and sulfide distributions showed a high degree of spatial heterogeneity under both light and dark conditions, and distributions were characterised by a complex mosaic of sediment zones dominated by either iron(II) or sulfide. However, there was a clear shift in overall redox conditions between light and dark conditions. During light deployments, iron(II) and sulfide concentrations were generally low throughout the rhizosphere, apart from a few distinct “hotspots” of high concentration. Whereas during dark deployments, high concentrations of iron(II) were sometimes measured in the near surface sediments and sulfide depth distributions migrated towards the sediment surface. Profiles of porewater ammonium and phosphate demonstrated an increase in ammonium concentrations under dark compared to light conditions. Surprisingly, despite the large changes in iron(II) distributions between light and dark conditions, phosphate profiles remained similar, indicating that adsorption/release of phosphate by iron(III) hydr(oxide) mineral formation and reduction was not a major factor regulating porewater phosphate concentrations in these sediments or that phosphate uptake by the seagrass roots persisted during the dark period. Overall, the results demonstrate that the photosynthetic activity of the seagrass played a significant role in regulating sulfide, iron(II) and ammonium concentrations in the rhizosphere, due to rates of radial oxygen loss and ammonium uptake by the roots and rhizomes being lower under dark compared to light conditions. This cyclic production and reduction of iron(III) hydr(oxides) in the rhizosphere may act as a buffering system preventing sulfide accumulation.

A Three-Stage Symbiosis Forms the Foundation of Seagrass Ecosystems

Seagrasses evolved from terrestrial plants into marine foundation species around 100 million years ago. Their ecological success, however, remains a mystery because natural organic matter accumulation within the beds should result in toxic sediment sulfide levels. Using a meta-analysis, a field study, and a laboratory experiment, we reveal how an ancient three-stage symbiosis between seagrass, lucinid bivalves, and their sulfide-oxidizing gill bacteria reduces sulfide stress for seagrasses. We found that the bivalve-sulfide-oxidizer symbiosis reduced sulfide levels and enhanced seagrass production as measured in biomass. In turn, the bivalves and their endosymbionts profit from organic matter accumulation and radial oxygen release from the seagrass roots. These findings elucidate the long-term success of seagrasses in warm waters and offer new prospects for seagrass ecosystem conservation.

Effects of Seagrass Rhizospheres on Sediment Redox Conditions in SE Asian Coastal Ecosystems

We examined the rhizosphere structure of 14 seagrass meadows (seven mixed, three Enhalus acoroides, two Zostera japonica, one Thalassia hemprichii, and one Halophila ovalis) in the Philippines and Vietnam and tested their effect on sediment redox potential by comparing the redox potential in vegetated vs unvegetated sediments. The effect of seagrass photosynthesis on sediment redox potential was tested in an E. acoroides meadow during a short-term (2-day) clipping experiment. In all the meadows, the centroidal depth (i.e., depth comprising 50%) of seagrass belowground biomass was within the top 15 cm sediment layer. Redox potentials in vegetated sediments tended to be higher than those in adjacent unvegetated ones; sediment redox potential anomaly ranged from −61 to 133 mV across the meadows. The centroidal depths of positive redox potential anomaly and seagrass root biomass were significantly correlated across the meadows investigated (type II regression analysis, slope = 0.90, lower confidence limit [CL] = 0.42 upper CL = 1.82, R 2 = 0.59, p < 0.01). Experimental removal of E. acoroides leaves resulted in a decrease in rhizosphere redox potential by 20 mV, further confirming the positive effect of seagrass roots and rhizomes on sediment redox potential and, thus, the general conditions for microbial processes in the coastal zone.

Aerobic heterotrophic microbial activity associated with seagrass roots: effects of plant type and nutrient amendment

The oxygen dynamics of seagrass roots implicitly influences the associated microbial activity. The oxic zone surrounding roots creates a distinct electrochemical gradient where the con- centration of oxygen is highest closest to the root tip and decreases sharply with distance away from the root. Microbes within each micro-niche utilise progressively less energy-favourable electron acceptors (e.g. oxygen, nitrate, sulphate). It is probable that oxygen-consuming bacteria would have greatest activity at or near the root tips of seagrass. This study assessed the potential of a community- level physiological profiling assay, based on a high throughput, microtitre-based oxygen sensor, to evaluate aerobic heterotrophs in seagrass ecosystems. Seagrasses were collected from shallow water within the Banana River Estuary, Kennedy Space Center. The sensitivity of the method for character- izing microbial communities associated with seagrasses was tested both spatially (i.e. rhizoplane, rhi- zosphere) and following imposed light stress (i.e. shading). Results showed that oxygen availability and competition for nutrients defined the microbial response to carbon substrates, with the fastest response and largest peaks for N-containing carbon sources (e.g. asparagine, natural extracts). Aer- obic microbial activity was higher for Halodule wrightii than Syringodium filiforme. Primary nitrogen and secondary phosphorus limitation was observed for microbial activity from the rhizoplane and rhi- zosphere of H. wrightii. Shading appeared to reduce nutrient limitation for H. wrightii and reduced aerobic microbial activity. The microbial activity and functional response varied between the rhizos- phere and rhizoplane, which suggests that microbial activity on this fine spatial scale needs to be considered when evaluating biogeochemistry associated with seagrass sediments.

Investigating biogenic heterogeneity in coastal sediments with two-dimensional measurements of iron(II) and sulfide

Environmental context. Microbial respiration generally occurs in distinct layers within coastal sediment, producing high porewater iron or sulfide concentrations, although this layering is dramatically modified by the activities of sediment-dwelling organisms. The present study describes use of a new technique to simultaneously measure two-dimensional concentrations of porewater iron and sulfide at millimetre resolution, allowing the patchiness of patterns of microbial respiration in sediment to be clearly observed. The measurements generally supported a conceptual model predicting the effects of animal burrows and seagrass roots on the porewater iron and sulfide distributions, although the addition of organic matter provided some unexpected observations that require further investigation. Abstract. One of the most powerful predictive tools in sediment biogeochemistry is the electron acceptor layering model, which describes the order in which oxidised compounds are reduced by successions of respiring microbial populations, and how this layering is influenced by benthic macro-organism activity. However, techniques allowing convenient determination of heterogeneous distributions of reduced substances, such as iron(II) and sulfide, have been lacking. A combined diffusive gradients in thin films–diffusive equilibrium in thin films technique was used to quantitatively measure the two-dimensional iron(II) and sulfide distributions at high resolution in the vicinity of various sediment features, including macrofauna burrows, particulate organic matter and macrophyte roots. Substantial heterogeneity was observed for both analytes in all probes, especially in the vicinity of seagrass roots and particulate organic matter. Measured distributions tended to follow the general patterns predicted by the tertiary electron acceptor layering model. However, there was unexpected overlap of sulfide and iron(II) distributions at the millimetre to centimetre scale in several samples from different sediments, notably the more complex sediments containing particulate organic matter and seagrass roots. The cause of such overlap is unclear and further study is necessary to elucidate how such distributions can occur.